The invention relates to a process for the preparation of catalyst with improved catalytic properties, particularly improved initial activity, initial selectivity and/or activity performance over time and/or selectivity performance over time.
The presence of certain species contained in the carriers of catalysts can be detrimental to the metal deposition process and/or catalyst performance and it is commonly believed that the concentration of these detrimental species must be controlled throughout the bulk of carrier. One way of controlling the amount of impurities through the bulk, although expensive, is by the use of purer raw materials. For example, U.S. Pat. No. 4,797,270 discloses water washing to reduce the sodium content of an alumina powder. The pH of the wash water may need to be adjusted for extraction of other metals and Japanese patent JP56164013 discloses the use of a low pH (acid) to extract uranium and thorium from a calcined xcex1-alumina raw material.
Several procedures in the art teach that washing after deposition of the catalytic metal is helpful. U.S. Pat. Nos. 4,361,504 and 4,366,092 suggest that ethylene oxide catalyst be water washed after the deposition of silver or silver/gold on the carrier. EP-211521 discloses washing of a catalyst with hot water to remove basic materials left on the catalyst from a silver impregnation process or the physical deposition of alkali metals. U.S. Pat. No. 4,367,167 discloses a process for preparing a supported catalyst wherein an impregnated support is immersed in an inert water immiscible organic solvent containing a dissolved aliphatic amine. U.S. Pat. No. 4,810,689 discloses depositing a silver compound, decomposing the silver compound to silver in the presence of an alkali metal compound, removing organic deposits by washing and introducing fresh alkali metal by impregnation during or after the washing stage. U.S. Pat. Nos. 4,186,106 and 4,125,480 disclose washing with an inert liquid after deposition of the catalytic metal and before deposition of a promoter material.
U.S. Pat. No. 4,908,343 teaches that it may be desirable to remove cations which are exchangeable with the alkali and alkaline earth metals contained in the impregnating solution, to allow for ease of repeatability in the use and reuse of the impregnating solution. No methods are taught for such removal; however, it is commonly known in the art that acids are highly effective cation removal solutions. U.S. Pat. No. 2,901,441 teaches washing a carrier with lactic acid then flushing with water.
The prior art remains concerned with the total amount of impurities; i.e., impurities throughout the bulk. Unfortunately, the impurity removal techniques taught typically attack the carrier itself. It has surprisingly been found that the metal deposition and/or catalytic properties of a catalyst may be greatly improved by controlling the purity of the surface of the carrier rather than the purity of the carrier bulk, such that the total amount of impurities may actually be high as long as the surface amount of impurities are maintained at a low level.
According to one embodiment of the invention, there is provided a process for improving the properties of a carrier, said process comprising selecting a carrier; and lowering a concentration of one or more ionizable species present on a surface of said carrier.
There is further provided a process for improving a carrier comprising selecting one or more materials; lowering a concentration of one or more ionizable species present in at least one of said one or more materials; forming a carrier comprising said one or more materials; and lowering a concentration of one or more ionizable species present on a surface of said carrier.
There is still further provided a process for improving the catalytic properties of a catalyst, said process comprising:
selecting a carrier;
lowering a concentration of one or more ionizable species on a surface of said carrier by a means effective in solubilizing the ionizable species and removing that species, or rendering the ionizable species insoluble, or rendering the ionizable species immobile;
optionally drying said carrier;
depositing a catalytically effective amount of one or more catalytically reactive metals on said carrier, thereby forming a catalyst precursor; and
optionally drying said catalyst precursor.
In another embodiment of the invention, there is provided a process for improving the catalytic properties of a catalyst, said process comprising:
selecting one or more materials;
lowering a concentration of one or more ionizable species present in at least one of said one or more materials by a means effective in solubilizing ionizable species and removing that species, or rendering the ionizable species insoluble, or rendering the ionizable species immobile;
forming a carrier comprising said one or more materials;
optionally lowering a concentration of one or more ionizable species on a surface of said carrier by a means effective in solubilizing and removing said ionizable species or rendering said ionizable species insoluble;
optionally drying said carrier;
depositing a catalytically effective amount of one or more catalytically reactive metals on said carrier, thereby forming a catalyst precursor; and
optionally drying said catalyst precursor.
In yet another embodiment of the invention there is provided a process for preparing a catalyst suitable for the vapor phase production of epoxides, said process comprising:
selecting a carrier;
lowering a concentration of one or more ionizable species present on a surface of said carrier;
optionally drying said carrier; and
depositing a catalytically effective amount of one or more catalytically reactive metals on said carrier.
The invention also provides for catalyst made by the processes of the embodiments herein described.
It has been found that carriers which have been treated to reduce certain undesirable ionizable species, particularly anionic species, which are present on the surface of the carrier provide catalysts with improved catalytic properties, such as activity, selectivity and activity and/or selectivity performance over time, when compared with the performance of catalysts made from carriers which have not been so treated. The process is believed to work to improve the properties of most catalysts, no matter how impure the bulk carrier material, compared to a catalyst made with an untreated carrier. Further, the process will work for organic or inorganic carriers.
The process is effective in improving one or more of the catalytic properties of a catalyst wherein a catalytically reactive metal is deposited or impregnated upon a carrier which contains ionizable species on its surface. xe2x80x9cImprovement in catalytic propertiesxe2x80x9d as used herein means the properties of the catalyst are improved as compared to a catalyst made from the same carrier which has not been treated to lower surface ionizable species. Catalytic properties include catalyst activity, selectivity, activity and/or selectivity performance over time, operability (resistance to runaway), conversion and work rate.
The process requires that the concentration of undesirable ionizable species present on the surface of the carrier be reduced. As used herein, the xe2x80x9csurfacexe2x80x9d of the carrier is that area of the carrier which may be measured by the standard method of Brunauer, Emmett and Teller (BET). Specifically, the surface of the carrier is the site at which reaction takes place. An xe2x80x9cionizablexe2x80x9d species is a species which is capable of being rendered ionic, where the term xe2x80x9cionicxe2x80x9d or xe2x80x9cionxe2x80x9d refers to an electrically charged chemical moiety.
Carriers are commonly inorganic materials such as, for example, alumina-, silica-, or titania-based compounds, or combinations thereof, such as alumina-silica carriers. Carriers may also be made from carbon-based materials such as, for example, charcoal, activated carbon, or fullerenes. Ionizable species typically present on the inorganic type carriers include sodium, potassium, aluminates, soluble silicate, calcium, magnesium, aluminosilicate, cesium, lithium, and combinations thereof. Of particular concern are the ionizable anionic species present on the surface, particularly ionizable silicates. Lowering the undesirable ionizable species concentration may be accomplished by any means (i) which is effective in rendering the ionizable species ionic and removing that species, or (ii) which renders the ionizable species insoluble, or (iii) which renders the ionizable species immobile; however, use of aggressive media is discouraged as these media tend to dissolve the carrier, extract too much material from the bulk, and generate acidic or basic sites in the pores. Acids, besides being an aggressive medium, will remove the cations on a carrier but are fairly ineffectual in removing the undesirable anions, such as silicates. Effective means of lowering concentration include washing the carrier; ion exchange; volatilizing, precipitating, or sequestering the impurities; causing a reaction to make the ionizable species on the surface insoluble; and combinations thereof. Examples of wash and ion exchange solutions include aqueous and/or organic solvent-based solutions which may also contain tetraethylammonium hydroxide, ammonium acetate, lithium carbonate, barium acetate, strontium acetate, crown ether, methanol, ethanol, dimethylformamide, and mixtures thereof. The formed carrier may be treated, or the materials used to form the carrier may be treated before the carrier is manufactured. When the carrier materials are treated before the carrier is formed, still further improvement may be seen by also treating the surface of the formed carrier. The carrier may be dried following the ionizable species reduction treatment.
To make a catalyst from the carrier, the carrier is typically impregnated with metal compound(s), complex(es) and/or salt(s) dissolved in a suitable solvent sufficient to deposit or impregnate a catalytically effective amount of metal on the carrier. As used herein, xe2x80x9ccatalytically effective amountxe2x80x9d means an amount of metal that provides a measurable catalytic effect. For example, a catalytically effective amount of metal when referring to an olefin epoxidation catalyst is that amount of metal which provides a measurable conversion of olefin and oxygen to alkylene oxide.
Further improvement in the catalyst properties are seen when the metal deposition is effected by contacting the carrier with an impregnation solution whose hydrogen ion activity has been lowered. xe2x80x9cHydrogen ion activityxe2x80x9d as used herein is the hydrogen ion activity as measured by the potential of a hydrogen ion selective electrode. As used herein, a solution with xe2x80x9cloweredxe2x80x9d hydrogen ion activity refers to a solution whose hydrogen activity has been altered by the addition of a base, such that the hydrogen ion activity of the altered solution is lowered compared to the hydrogen ion activity of the same solution in an unaltered state. The base selected to alter the solution may be chosen from any base or compound with a pKb lower than the original impregnation solution. It is particularly desirable to chose a base which does not alter the formulation of the impregnation solution; i.e., which does not alter the desired metals concentration in the impregnation solution and deposited on the carrier. Organic bases will not alter the impregnation solution metals concentrations, examples of which are tetraalkylammonium hydroxides and 1,8-bis-(dimethylamino)-naphthalene. If changing the metals concentration of the impregnation solution is not a concern, metal hydroxides may be used.
When the impregnation solution is at least partially aqueous, an indication of the change in the hydrogen activity may be measured with a pH meter, with the understanding that the measurement obtained is not pH by a true, aqueous definition. xe2x80x98xe2x80x9cMeasured pHxe2x80x9dxe2x80x99 as used herein shall mean such a non-aqueous system pH measurement using a standard pH probe. Even small changes in the xe2x80x9cmeasured pHxe2x80x9d from the initial impregnation solution to that with added base are effective and improvements in catalytic properties continue as the xe2x80x9cmeasured pHxe2x80x9d change increases with base addition. High base additions do not seem to adversely affect catalyst performance; however, high additions of hydroxides have been seen to cause sludging of the impregnation solution, creating manufacturing difficulties. When the base addition is too low, the hydrogen ion activity will not be affected. The hydrogen ion activity lowering procedure is also quite effective when used by itself; i.e., when no ionizable species concentrations are lowered prior to impregnation.
The impregnated carrier, also known as a catalyst precursor, may be dried in the presence of an atmosphere which also reduces the catalytic metal. Drying methods known in the art include steam drying, drying in an atmosphere with a controlled oxygen concentration, drying in a reducing atmosphere, air drying, and staged drying using a suitable ramped or staged temperature curve.
By way of example, the process will be described in more detail for a catalyst suitable for the vapor phase production of epoxides, also known as an epoxidation catalyst.
First, a carrier is selected, in the case of an epoxidation catalyst the carrier is typically an inorganic material, such as for example, an alumina-based carrier such as xcex1-alumina. The concentration of undesirable ionizable species present on the surface of the carrier are reduced to create a xe2x80x9ccleansedxe2x80x9d carrier. Or, alternatively, the concentration of ionizable species in the materials used to make the carrier may be reduced prior to formation of the carrier. If the carrier raw materials are treated, the formed carrier may be retreated for further improvement.
Ionizable species present on an xcex1-alumina carrier, for example, typically include sodium, potassium, aluminates, soluble silicates, calcium, magnesium, aluminosilicates, and combinations thereof. It has been found that silicates, and certain other anions, are particularly undesirable ionizable species in an epoxidation catalyst. The solubilization rate of silicates may be measured by inductively coupled plasma (ICP) techniques and the amount of silicon species on a surface may be measured by x-ray photoelectron spectroscopy (XPS); however, since sodium is soluble in the same solutions that silicates are soluble in, the solubilization rate of sodium becomes a simpler check of the ionic species removal. Another measurement technique is to measure the change in electrical conductivity of the treatment solution.
The concentration of the undesirable ionizable species may lowered by any means which is effective in rendering the ionizable species ionic and removing that species, or rendering the ionizable species insoluble, or rendering the ionizable species immobile. Means effective in lowering the concentration of the undesirable ionizable species on the surface include washing, ion exchange, volatilization, precipitation, sequestration, impurity control and combinations thereof. Cleansing of an alumina-based carrier may be efficiently and cost-effectively accomplished by washing or ion exchange. Any solution capable of reducing the concentration of the undesirable ionizable species present, particularly the anionic ionizable species, and most particularly ionizable silicates, may be used.
After the concentration of the surface ionizable species are lowered, the carrier is optionally dried. When aqueous or organic solvent washing is used, drying or some similar method is recommended to displace the wash solution from the carrier pores. The carrier is now ready for a catalytically reactive metal to be deposited or impregnated thereon. For further improvement in catalytic properties, the hydrogen ion activity of the impregnation solution is optionally lowered, such as by the addition of a base. The typical impregnation solution for an epoxidation catalyst begins quite basic, so a strong base is used to further lower the hydrogen ion activity. Examples of strong bases include alkyl ammonium hydroxide such as tetraethylammonium hydroxide, and metal hydroxide such as lithium hydroxide and cesium hydroxide. In order to maintain the desired impregnation solution formulation and metal loading, a organic base such as tetraethylammonium hydroxide is preferred. Base additions in these systems typically result in a xe2x80x9cmeasured pHxe2x80x9d change ranging up to about 3 pH units, realizing that the xe2x80x9cmeasured pHxe2x80x9d is not a true pH since the impregnation system is not aqueous.
If an excess of impregnation solution is used, the impregnated carrier is subsequently separated from the solution before the deposited metal compound is reduced. Promoters, components which work effectively to provide an improvement in one or more of the catalytic properties of the catalyst when compared to a catalyst not containing such components, may also be deposited on the carrier either prior to, coincidentally with, or subsequent to the deposition of the catalytically reactive metal.
The resulting epoxidation catalysts just described are used for the vapor phase production of epoxides. A typical epoxidation process involves loading catalysts into a reactor. The feedstock to be converted, typically a mixture of ethylene, oxygen, carbon dioxide, nitrogen and ethyl chloride, is passed over the catalyst bed at pressure and temperature. The catalyst converts the feedstock to an outlet stream product which contains ethylene oxide. Nitrogen oxides (Nox) may also be added to the feedstock to boost catalyst conversion performance.
Other embodiments of the invention provide catalysts made by the processes just described.